A conventional CRT comprises an evacuated envelope having a viewing screen coated with an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen and a color selection structure or shadow mask comprising a thin multi-apertured metal sheet precisely disposed between the screen and the beam-producing means. The shadow mask shadows the screen and the differences in convergence angles permit the transmitted portions of each beam to selectively excite phosphor elements of the desired emission color.
The conventional CRT shadow mask is typically manufactured by first coating a photoresist on a thin metal plate made of Invar or aluminum-killed steel. The plate is then exposed to light, and developed and etched to form a plurality of holes therein. Thereafter, the plate formed with the holes is annealed using a heat treating process in a hydrogen atmosphere at a high temperature, thereby removing residual stress and providing softness to the plate. The plate is then formed into a predetermined mask shape by the use of a press, after which the plate is cleaned to remove all contaminants from its surface such as fingerprints, dust and other foreign substances. Finally, a blackening process is performed on the shaped plate to prevent doming of the same, thereby completing the manufacture of the shadow mask.
The shadow mask acts as a bridge between electron beams emitted from three electron guns (means for producing three convergent electron beams) and red, green and blue phosphor pixels formed on the screen because it ensures that the electron beams land on the correct phosphor pixels. Accordingly, any deviation of the shadow mask from its original position acts to mis-direct the electron beams to excite unintended phosphor pixels.
The shadow mask can be moved away from its originally-set position in the CRT if it receives external shock or vibrations such as by the loud audio from speakers in the TV set. As the resut electron beams passing through moved shadow mask will land on the wrong phosphor pixel causing deteriorated color purity. This will be described in more detail hereinbelow.
FIG. 1 shows a partial sectional view of a conventional CRT. It shows a shadow mask 6 mounted to a side wall of the panel 1. More specifically a mask frame 5 joined to a periphery of the shadow mask 6 is coupled to a spring 4, which is in turn connected to a stud pin 3 protruding from the side wall of the panel 1. When the CRT receives a substantial external shock or vibrations, the shadow mask 6 is shaken and moves away from its initial position to a deviated position 7. As the result the electron beams 10 emitted from the electron gun 11 will pass through an unintended aperture of the shadow mask 6 resulting in the excitation of the wrong phosphor pixel. This is perceived as shaking of a displayed picture and thus causes a reduction in color purity and other picture quality problems.
Furthermore, in the case where the CRT receives an extreme shock, such as when it is dropped, it is possible for the shadow mask 6 to be deformed. An example of this is shown in FIG. 2 in which a deformed area 12 is illustrated. Needless to say, spurious colors would appear.
In an attempt to solve the above problem, the mask was heat-treated to improve its tensile strength and softness. However, since the heat-treating of a shadow mask increases its modulus of elasticity in the skirt portion of the shadow mask, an angle of the bend in the skirt portion made while press-forming the metal plate used to make the shadow mask is not the intended .theta. degrees, but rather a .theta.+.DELTA..theta. degrees as a result of the spring back phenomenon of the skirt portion.